Field of the Invention
The present invention relates generally to memory devices and more specifically to a programmable read-only memory structure (PROM) and method of operation.
With the advent of the use of elements was switching devices and more specifically as a memory cell, interest was developed in the use of amorphous semiconductive layers as switching and memory elements. "Switching In Elemental Amorphous Semiconductors" by Charles Feldman and Kishin Moorjani, in the Journal of Non-Crystalline Solids 2, (1970), pages 82-90, reports the ability of amorphous forms of silicon and geranium to operate as switching devices equivalent to the semiconductor glasses. The incorporation of this principle as memory elements described in "Read-Only Store Memory Element" by M. H. Brodsky, R. C. Dockerty and K. Weiser, IBM Technical Disclosure Bulletin, Vol 13 No. 11, Apr. 11, 1971 on page 3223. The Brodsky et al disclosure increases the local temperature to a point where the diffusant or alloy material diffuses into and/or alloys with the amorphous film to create additional conductive paths in the silicon in addition to the crystallization of the amorhpous semiconductor. This disclosure produces a polycrystalline structure which is difficult to control so as to reproduce reliably because of the diffusing or alloy mechanism. Thus there exists a need to provide a structure and a mechanism which will allow the use of semiconductor amorphous films as reproducible memory storage elements in a read-only memory structure.
Recent research has shown that epitaxial layers of silicon can be grown on single-crystal silicon substrates from an amorphous silicon layer by means of a solid state reaction. This solid-phase epitaxial growth (SPEG) of silicon is reported in "Dissociation Mechanism For Solid-Phase Epitaxy of Silcion in the Si (100)/Pd.sub.2 Si/Si (amorphous) System"; R. Pretorius, Z. L. Liau, S. S. Lau and M. A. Nicolet, Applied Physics Letters, Vol. 29, No. 9, Nov. 1, 1976, Pages 598-600. The present invention uses the SPEG mechanism in a memory structure similar to that described by Brodsky et al to provide a memory element which is most reproducible because of the epitaxy and capable of storing multiple signal levels.